Refactor removed ability to handle Enter & arrow keys on grid

Refactor: https://github.com/angular-ui/ng-grid/pull/3120 says this:

The problem is that viewPortKeyDown does not get called for navigation keystrokes (specifically Enter and Left/Right Arrow). Previous to this refactor, we could attach an ng-keydown in a cell template to handle these keys.

We desperately need this, as the Enter key lets us submit the form the user is on and the left/right arrow keys let us scroll the grid immediately, irrespective of which column has focus.

I can try to submit a bug fix if that's preferred, though I might not implement it properly...